When coking coal, aromatic hydrocarbons are released as a component of the coke-oven gas created. In order to be able to exploit the aromatic hydrocarbons, and not to release them into the environment, they are usually washed out of the coke-oven gas during processing of the coke-oven gas mostly after the separation of tar and ammonia. In practice, a wash oil that is mainly a tar oil fraction is used as the wash liquid, which is created by the processing of black coal. According to the main aromatic hydrocarbons, benzene, toluene, and xylene contained therein, the process step is generally also called BTX or benzene washing. The aromatic hydrocarbons are also called raw benzene, the part of raw benzene being typically between 20 and 40 grams per normal cubic meter (Nm3) depending on the coal and process control used for the coking process. The raw benzene typically has between 55 to 75% benzene, 13 to 19% toluene, and between 5 to 10% xylene. Furthermore, the coke-oven gas also has multicyclic aromatic hydrocarbons, such as particularly naphthalene, which may be absorbed by the wash oil. Furthermore, the coke-oven gas contains contaminations, particularly H2S, HCN, NH3, and organic sulfur compounds. A typical composition of a coke-oven gas, for example, comprises the following:
H2 54 to 62% by volume
CH4 23 to 28% by volume
CO 6.2 to 8% by volume
H2S approximately 1.5 g/Nm3 
HCN approximately 1.5 g/Nm3 
NH3 7 g/Nm3 
SORG approximately 0.5 g/Nm3 
BTX up to 40 g/Nm3 
naphthalene up to 2 g/Nm3 
The methods for BTX washing, with regard to main features, have been used in an unchanged manner for decades, and are described, for example, in the technical literature of O. Grosskinsky, “Handbook of Cookery,” volume 2, edition 1958, pages 137ff. BTX washing is carried out in one or more successively arranged washers, intimate contact between the coke-oven gas and wash oil as the wash liquid being ensured for the absorption of the aromatic hydrocarbons by the wash oil. Intimate contact can be achieved both by a fine atomization of the wash oil and by thin oil films. The combination of an irrigation system, as well as of cookers, filling bodies, or other installed equipment is particularly advantageous, the oil droplets coming from the irrigation system being spread into an oil film having a surface that is as large as possible. The solubility of benzene, toluene, and xylene is particularly dependent on the vapor pressure of the various components, which is why the wash oil is supplied to the scrubber at comparatively low temperatures, preferably approximately room temperature. On the other hand the wash oil must also have sufficient flowability and low viscosity so that it easy to atomize and may form a large surface. The wash oil enriched with aromatic hydrocarbons collecting at the bottom of the scrubber is removed, the raw benzene being subsequently stripped from the wash oil with high-temperature water-vapor stripping. After cooling the wash oil is then returned to the scrubber. In order to achieve extensive washout of raw benzene with a coke-oven gas flow that is as large as possible, the wash oil is fed to the scrubber in excess. In order to be able to carry out BTX washing with the amounts of coke-oven gas occurring in modern coke ovens, large amounts of wash oil are necessary.
By way of trial, fossil diesel oil may also be used as a wash liquid. However, for this purpose it has been found that sticky residues and a rubber-like consistency occur in the wash oil and are precipitated during the stripping using water vapor at the process temperature. The residues result from the copolymerization reactions in which the contaminations contained in the coke-oven gas, particularly HCN and H2S as complexing agents are involved. In order to separate the sticky, rubber-like precipitations centrifuges must be provided in the washing fluid cycle. It is further necessary to discharge a relatively large partial flow of the wash liquid from the cycle, and replace it with fresh wash liquid, if fossil diesel oil is used as the wash liquid. The residues separated by the centrifuge and the fossil diesel oil discharged from the cycle must be disposed of. The measures described are extensive, and make the process more cost-intensive. The complex reactions are temperature dependent. With the high temperatures of over 120° C. that are necessary in order to be able to economically operate the regeneration of the wash liquid by vapor stripping, the amount of sticky residues occurring is so large that the process can no longer be operated.